It's so hot that Australian John Millman asked for an unusual timeout in the middle of gameplay during his quarterfinal match with No. 6-seeded Novak Djokovic on Wednesday night.

Millman approached the net early in the second set to ask for a quick break to change his shorts. The 29-year-old said he was having trouble getting tennis balls out of his pocket because of the heat and humidity in Arthur Ashe stadium.

Millman explains to Djokovic that he needs to change because of the heat in Arthur Ashe stadium. Matthew Stockman/Getty Images

It's so humid inside Arthur Ashe that the Open implemented an "extreme heat policy" for the tournament this year. Players are allowed a 10-minute break between the third and fourth sets. They are also allowed medical timeouts for "heat-related illness."

Djokovic embraces a very sweaty Millman after the end of the quarterfinal match late Wednesday evening. Julian Finney/Getty Images

In an interview with ESPN after the match, Djokovic said the humidity in the stadium didn't make for easy conditions to play in. He also commented on Millman leaving the court mid-set to change and how he was fine with it.

"He [Millman] was apologizing, actually. He was saying that he felt very sorry that he has to leave the court and change," said Djokovic. "I said, 'Man, go ahead. I'm actually OK sitting down for 10 minutes, you know, just relaxing. I needed that rest. It was great."

Djokovic certainly appeared to be enjoying himself during a much-needed break where he took his shirt off. AP Photo/Frank Franklin II

INSIDER was at the event Monday evening for Millman's history-setting match against Roger Federer. That night, the conditions inside the stadium were much warmer than they were outside the seating area in the concession.

During a press conference after his match Monday, Federer also commented on the heat inside Arthur Ashe, suggesting poor air circulation in the stadium may be due to the roof that was added in 2016.

"I just thought it was very hot tonight... It was just one of those nights where I guess I felt I couldn't get air. There's no circulation at all [in Arthur Ashe]," said Federer, explaining his performance Monday night.

Federer addresses what went wrong during his Monday night match with Millman. ESPN

"I do believe since the roof is on there is no air circulation in the stadium," Federer added. "I think just that makes it a totally different US Open, plus conditions, maybe we're playing slower this year on top of it."

As the US Open heads into its semifinal matches Thursday, the weather in New York City should start to get cooler. The weekend forecast calls for temperatures in the 70s, which may offer some much-needed relief to players at the tournament.